Recent control circuits are integrated and have many functions by combining analog circuit sections and digital circuit sections or by increasing digital circuit sections. To facilitate easy confirmation of functions of the digital circuit sections, that is, to improve a test coverage or testability, the control circuits are configured to operate in a test mode to perform function tests. Each control circuit mounted in a vehicle, however, is subjected to very harsh operating conditions. As a result, when the control circuit is erroneously switched to the test mode due to mechanical causes such as vibrations or electric noises during a travel of the vehicle, the control circuit is disabled to operate properly.
Further, even when a vehicle is at rest, the control circuit should be protected from being erroneously switched to the test mode so that the dark current of the control circuit may be reduced or erroneous operation of a system may be prevented. When it is erroneously switched to the test mode, a fail safe operation is needed to switch it back to a normal mode.
In JP-8-161001A for instance, therefore, a test mode operation check signal is stored in a nonvolatile memory, and the test mode is executed only when a key operation for a test mode operation is made under the condition that the check signal indicates permission of the test mode operation.
It however requires an additional work of storing the check signal in the nonvolatile memory each time. It is possible only in the case that the nonvolatile memory is rewritable from an external side. Further, it is still likely that a control circuit is erroneously switched to the test mode even if the check signal is stored.